in eyes post trauma could be due to a simple SCH( sub conjunctival hemorrhage) or traumatic Iritis
with or without retinal involvement depending on the nature of injury
If you see the redness localized in white part of the eye,not associated with pain / sensitive to light / blurring of vision ,then it must be just a SCH which will resolve by itself in 2 weeks time.
But, if the redness is diffuse throughout the eye, along with pain on blinking,photophobia
and decreased vision then it is due to inflammation of the inner layers of the eye, which needs a dilated retinal examination to rule out any serious damage and management.
I hope this helps and I wish your son a speedy recovery.